1. Field of the Invention
The present invention relates to electrophotographic photosensitive members, process cartridges, and electrophotographic apparatuses. In particular, the present invention relates to an electrophotographic photosensitive member using a charge transfer material synthesized by a specific method, a process cartridge and an electrophotographic apparatus which include the electrophotographic photosensitive member, and a process for producing the electrophotographic photosensitive member.
2. Description of the Related Art
In recent years, laminate-type electrophotographic photosensitive members, each having a photosensitive layer including a charge generating layer and a charge transfer layer, have been proposed. The electrophotographic photosensitive members having the laminate structure have improved in sensitivity to visible light, charge retention, and surface strength. Many organic compounds have been proposed as charge transfer materials. For example, Japanese Unexamined Patent Application Publication No. 52-72231 discloses pyrazoline compounds, Japanese Unexamined Patent Application Publication No. 55-52063 discloses hydrazone compounds, Japanese Unexamined Patent Application Publication Nos. 54-58445 and 57-195254 disclose triphenylamine compounds, and Japanese Unexamined Patent Application Publication Nos. 54-151955 and 58-198043 disclose stilbene compounds. Since triarylamine compounds having a triphenylamine structure have superior electrophotographic characteristics, such as easy molecular design and high hole mobility, many novel proposals have been disclosed.
However, electrophotographic photosensitive members using these triarylamine compounds as charge transfer materials do not always have adequate sensitivity and still require improvements in potential variation when being repeatedly used and image defects at low humidity and high humidity.
The characteristics of the electrophotographic photosensitive member are affected by not only the structure of the charge transfer material but also the purity thereof. In particular, it is known that the variation of the rest potential is greatly affected by the impurities in the charge transfer material. Thus, it is preferable that the purity of the charge transfer material used in the electrophotographic photosensitive member be higher and the impurity content be lower. It is considered that the impurities trap holes, which are carriers in the charge transfer layer, and inhibits carrier transfer and that the accumulated holes form space charge, which is a factor-of variations of the resist potential. Thus, it is preferable that the impurity content be lower.
In conventional production processes of charge transfer materials, the final stages of the processes include purification treatments, such as recrystallization and column chromatography. However, recrystallization does not sufficiently remove impurities and results in a low yield of the final product. Column chromatography uses expensive chromatograph-grade silica gel or alumina and large amounts of hazardous flammable organic solvents, having cost and safety problems.
An arylamine compound used in the charge transfer material is synthesized by the condensation reaction of the corresponding aryl halide with an amine compound. For example, synthesis from the corresponding iodobenzene and an amine compound in the presence of a copper catalyst (Ullmann reaction) is known (refer to “Daiyuukikagaku”, vol. 16, p. 52 (1959), Asakura Shoten; and “Yuukikagaku Koza”, vol. 3, p. 66 (1983), Maruzen). This reaction, however, requires a large amount of copper catalyst, a high reaction temperature, and a prolonged reaction time. Thus, this reaction results in a low arylamine yield and forms byproducts, such as colored impurities and decomposition products, which adversely affect electrophotographic characteristics, and thus requires much purification cost.
Stephan L. Buchwald et al. discloses synthesis of arylamines from aryl halides and amines in the presence of a catalyst including a phosphine and a palladium compound (Tetrahedron Letters, Vol. 36, No. 21, 3609 (1955); and J. Am. Chem. Soc., Vol. 120, 9722 (1988)). Since this reaction proceeds under a relatively mild condition, the impurity yield is significantly low compared to the Ullmann reaction. John F. Hartwig et al. also discloses a similar reaction (J. Org. Chem., 61, 1133 (1996)).
Moreover, as synthesis of triarylamines by applying these methods, Japanese Unexamined Patent Application Publication Nos. 10-139742 and 10-310561 disclose synthesis using a catalyst including a trialkylphosphine and a palladium compound. Although these methods have advantages, such as a relatively low reaction temperature and a shortened reaction time, use of expensive trialkylphosphines causes increased production cost. Moreover, these methods causes new problems, for example, insufficient stability of the preserved catalyst and possibility of spontaneous combustion.
The present inventors have concentrically investigated means for solving the above problems and have found that compounds having a specific structure among phosphine compounds used for synthesizing triarylamines by the reaction proposed by Stephan L. Buchwald exhibit low cost, superior preservation stability, and high safety and that electrophotographic photosensitive members using these triarylamines exhibit stable potentials during endurance testing and environmental stability.